The First Six Weeks of Pregnancy, Explained

By the time a woman is considered six weeks pregnant, she would have had two weeks, at most, to realize it.

That’s because the gestational age of a fetus is counted not from the moment that sperm fertilizes an egg or from the moment you have a positive pregnancy test, but weeks earlier, on the first day of the previous menstrual cycle. This means that just two weeks after a missed period, a woman is six weeks pregnant, said Dr. Dawnette Lewis, director of Northwell Health’s Center for Maternal Health in New York and a maternal fetal medicine specialist.

That’s if someone has a typical menstrual cycle, which lasts about four weeks. But several factors — including stress, perimenopause and certain health conditions — can make the menstrual cycle so unpredictable that it could take longer than six weeks for someone to realize they might be pregnant.

“People come in and they’re like, ‘I’ve always had irregular periods and I just thought I was gaining weight’ and lo and behold, they are pregnant,” said Dr. Shruthi Mahalingaiah, a fertility doctor at Massachusetts General Hospital and professor of environmental and reproductive health at Harvard T.H. Chan School of Public Health.

The question of how soon it’s possible to detect and confirm a pregnancy has come into focus as new abortion restrictions have been enacted in 21 states, including in Florida, where a ban on abortion after six weeks is set to take effect May 1, making it the third state to do so.

We asked experts to explain how the first six weeks of pregnancy unfold, and what factors might make a pregnancy hard to detect.

Weeks 1 to 2:

If an egg isn’t fertilized, the uterus sheds its lining. The first day of bleeding is considered Day 1 of a menstrual cycle. Typically, this bleeding lasts between three and seven days, Dr. Lewis said.

As the bleeding subsides, estrogen and another hormone, called follicle stimulating hormone or F.S.H., begin to rise. Together, they nudge the ovaries to produce a mature egg and create a fresh uterine lining for a potential fertilized egg.

At or around Day 14 of a cycle, the ovary would typically release an egg. If an egg is released and sperm is present, fertilization can occur in the fallopian tube a day or two after ovulation.

Weeks 3 to 4:

If an egg has been fertilized, it becomes an embryo, which typically travels into the uterus and implants itself on the lining. This can take around a week or more, Dr. Belmonte said.

Once this happens, the body begins producing a hormone called human chorionic gonadotropin, also known as the pregnancy hormone. This tells the ovaries to hold off producing another egg.

At implantation, a woman would technically be considered about four weeks pregnant, based on how gestational age is calculated. But she would likely not have any physical symptoms of pregnancy, like nausea or fatigue. HCG levels may also be too low to be detected by an at-home pregnancy test; it is only detectable about a week after implantation, said Dr. Michael Belmonte, an obstetrician-gynecologist and fellow at the American College of Obstetricians and Gynecologists.

It is impossible to know if the embryo will keep developing normally or whether it even implanted in the uterus, said Dr. Mahalingaiah. About half of fertilized embryos do not end up implanting at all. In an estimated 2 percent of pregnancies, the embryo implants elsewhere, like the fallopian tube. This is known as an ectopic pregnancy and can be life-threatening, requiring medical or surgical intervention.

Week 5:

If you are pregnant, this is, roughly speaking, the earliest that you might be able to detect a pregnancy at home. If you have a regular four-week cycle, have missed your period and think you might be pregnant, you might take a urine test at home at the tail end of this week, which would pick up elevated hCG levels.

But there are several reasons to expect a period to be irregular and not think to take a pregnancy test at this point. Polycystic ovarian syndrome, which affects as many as five million women in the United States, can disrupt ovulation and cause irregular menstrual cycles, as can other endocrine disorders like thyroid disease. Significant amounts of stress in the first two weeks of your cycle can delay ovulation or menstruation, said Dr. Mahalingaiah, as can a bad case of the flu. Those who are in perimenopause, the transition to menopause, experience increasingly irregular ovulation as they approach the end of their reproductive years. This means that their menstrual cycles can change in length every month, often by several weeks.

Even if you did suspect you were pregnant around week 5, a health care provider would not be able use an ultrasound to confirm that a pregnancy is viable, because there would be no fetal cardiac activity yet. The pregnancy may also turn out to have been what is known as a biochemical pregnancy, Dr. Mahalingaiah said. When this happens, someone might test positive on an early home pregnancy test, but the fetus doesn’t develop further. This results in a pregnancy loss that simply looks like a late period. Some research suggests an estimated 22 percent of pregnancies end this way.

Week 6:

This is approximately when an ultrasound could pick up fetal cardiac activity, which can sound like a heartbeat. Some states require an ultrasound before a woman can obtain an abortion, including a medication abortion. Florida requires two in-person visits, 24 hours apart, including an ultrasound. The new law also prohibits telehealth providers from prescribing medication to end a pregnancy.

That timing, Dr. Belmonte said, leaves women with “a very small window” in which to confirm a pregnancy and obtain an abortion.